1. Field of the Invention
This invention relates in general to an ultrasound transducer that has a two-dimensional (2-D) array of transducer elements, and in particular to a 2-D transducer array for acquiring imaging data in different scan planes.
2. Description of the Related Art
The value of ultrasonic imaging as a diagnostic tool is undisputed, and as the power of ultrasound imaging systems increases, so too do the number of applications of the technology. Examples of such applications include the determination of the size and location of tumors and arterial occlusions. Another application that shows growing promise is in the area of cardiac imaging. As in many areas of diagnostics, however, physicians (and of course, patients themselves) prefer minimally, or, ideally, completely non-invasive techniques. This reduces the desirability of intracardiac imaging transducers, which are carried at the end of a catheter that is threaded into the heart via a major vein in the arm, neck or groin. There are, accordingly, several known systems in which both specialized and multi-purpose ultrasound systems are used for cardiac imaging via an externally applied probe.
One requirement that restricts the use of general ultrasound transducers is that some cardiac exams need the acquisition of two views from a common window. These views (two-chamber, four-chamber or short/long axis) are often approximately perpendicular to one another, which means that the operator must twist the probe in order to acquire both views. For example, multi-planar ultrasound scans are found in some systems used to determine cardiac output.
One way to achieve a display of real-time imaging planes, as well as calculate cardiac output reliably, is to design and use a 3-D ultrasound scanner. Unfortunately, the complexity and cost of such a real-time 3-D (sometimes referred to as 4-D) system are formidable. For example, a 2-D array is commonly used to interrogate the volume in real-time. Unfortunately, the element count for such a 2-D array is usually much larger (typically, well over 4000 elements) than for a 64-element 1-D array in order to achieve the same imaging performance.
To circumvent this problem, most existing real-time 3-D systems use a sparsely sampled aperture, which reduces not only the signal-to-noise ratio (SNR) of the transducer, but also the contrast and detail resolution relative to what a linear array can achieve. The transducer""s performance can be improved by using a complex multilayer ceramic array in transmit and receive, a receive preamplifier in the transducer assembly and/or more transducer elements. Even with these techniques, a sparsely sampled aperture still requires a significant number of elements, typically at least ten times greater than a conventional 64-element array. Because of the large number of elements that must be addressed in a sparse 2-D array, multiplexing (synthetic aperture) is sometimes used, but this reduces the frame rate.
What is needed is an ultrasound probe that is able to offer 3-D, real-time imaging information, that can shorten examination time, and that avoids the complexities found in a conventional real-time 3-D scanner. The probe should use a common array face in order to maintain a small footprint, which is particularly important for cardiac applications. Moreover, the probe should have contrast and detail resolution comparable to what can be achieved using a linear array. This invention provides such a probe.
An ultrasonic imaging array according to the invention comprises two independent, interleaved linear subarrays that occupy a common array face. The subarrays are independently steerable and focusable in different imaging planes. One advantage of the preferably embodiment of the array according to the invention is that each subarray is able to form an independent, unswitched aperture.
In the preferred embodiment of the invention, each subarray comprises a plurality of elements and each element comprises at least one subelement. Each subelement is quadrilateral and has a diagonal and adjacent subelements in each element are electrically connected via an interconnect portion. The interconnect. portion that connects each pair of adjacent subelements in each element is preferably substantially linear and is aligned with the diagonals of the adjacent subelements. In the preferred embodiment of the invention, the diagonals of all subelements in each element are aligned and the interconnect electrically connecting the subelements in each element is linear over the extent of the array
A multi-layer flex circuit is preferably used to connect the various element xe2x80x9cinterconnects to the external control and processing system. In this case, the interconnects for a first one of the subarrays are patterned as first linear traces onto a first separate layer of the flex circuit and extend to a first edge of the array face. The interconnects for a second one of the subarrays are then patterned as second linear traces onto a second separate layer of the flex circuit and extend to a second edge of the array face that is different from the first edge. A connector may then be provided for each subarray, the connector for each subarray being connected to the interconnects of the respective subarray along the respective edge.
The invention also allows the array to be connected in a multidimensional configuration such as, for example, 1.5 D or 1.75. The subelements in each element are in such embodiments grouped into a plurality of groups, the groups in each element of each respective subarray having the same relative position within the subarray. The subelements in each group are then electrically connected, whereby each subarray operates as a multidimensional array, with a dimension greater than one and less than two. For example, a first group may consist of a central plurality of subelements and a second group may consist of the plurality of subelements located on either side of the first group.
The array preferably comprises first and second subelements, in which the first subelements comprise a first one of the subarrays; the second subelements comprise a second one of the subarrays; edges of the subelements extend in a first and a second direction; and the first and second subelements are arranged in a pattern in which they alternate in both the first and second directions.
The invention also provides an acoustic lens mounted over the array face, which defines a single, common aperture, in which each subarray comprises a plurality of elements, the elements in a first one of the subarrays extend in an azimuth direction, and the elements in a second one of the subarrays extend in an elevation direction, which is orthogonal to the azimuth direction. The lens is preferably curved in both the azimuth and elevation directions, which allows the subarrays to be independently focusable.
Instead of using the lens, the subelements may be diced from ceramic so as to be convex-concavexe2x80x9d, that is, curved in both the first and second dimensions, the first and second directions being orthogonal, whereby the subarrays may be focused in both the first and second directions.
Because the subarrays according to the invention may be controlled independently, they may be operated so as to generate simultaneous transmit beams and/or receive simultaneous echo signals within the region of interest along the different imaging planes. In embodiments of the invention in which dual transmit beams are generated, the transmit beams may have different waveforms. It is also possible according to the invention to generate a transmit beam in one imaging plane and receive from the orthogonal plane; indeed, this method is used in one embodiment of the invention that makes possible 3-D imaging from within an entire volume of interest.
The invention is also able to generate from the received echo signals and simultaneously display for a user two orthogonal cross-sectional B-mode images of the region of interest. In this case, where the region of interest includes a body structure, the invention also comprises a method in which the user moves the array to a plurality of positions and, at each position, traces a periphery of the body structure as displayed in a first one of the orthogonal cross-sectional B-mode images. A system according to the invention then measures the distance of movement of the array as a function of sequentially generated second ones of the B-mode images, which are orthogonal to the first B-mode images. It then also calculates a volume of the structure as a function of the products of areas within the traced periphery and the corresponding measured distances, summed over all the positions of the array.